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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

384

FATTY SUBSTANCES  (GENERAL METHODS)

The ester number of stearolactone being 198-9, the mixture examined con-
tains

198-9
of stearolactone.

The determination of the lactones is of some importance with certain indus-
trial products of fatty matters, e.g., turkey red oil and distilled stearine, the
latter especially being rich in stearolactone, i.e., in the internal anhydride of
hydroxystearic acid.

17. Determination of the Glycerine

The simplest method of determining if glycerine is present in a substance
is to heat a little of the latter to boiling with a few crystals of potassium
bisulphate : in presence of glycerine, unpleasant, irritating odours of acrolein
are evolved, while a strip of filter-paper soaked in concentrated sodium
nitroprusside solution containing a little piperidine is dyed an intense blue
if placed in the mouth of the test-tube.

The glycerine in fatty substances may be determined indirectly, knowing
that in the saponification of neutral fats I mol. of glycerine (92 grams)
corresponds with 3 mols. of potassium hydroxide (168-3 grams). Multipli-
cation of the ester number (see g, above) by 0-05466 also gives the glycerine
content. This method is, however, only applicable when the fat does not
contain higher alcohols, unsaponifiable substances, etc.

To determine the glycerine directly, the fatty substance (20 grams) is
saponified in the ordinary manner (see 5 : Saponification), the soap decom-
posed by an acid, the fatty acids separated by filtration, and the glycerine
in the aqueous filtrate determined by one of the methods given in the next
chapter for the quantitative analysis of glycerine.

18. Determination of the Solid and Liquid Fatty Acids

The fixed (or insoluble) fatty acids entering into the composition of
fatty substances may be divided into two principal groups : solid, saturated
acids belonging to the acetic acid series and represented especially by stearic
and palmitic acids (sometimes also by arachic and lignoceric acids), and
liquid, unsaturated fatty acids belonging to the acrylic series and to other
less hydrogenated series and represented especially by oleic acid and often
also by linoleic, linolenic and ricinoleic 'acids.

The best method for separating these two groups of acids is based on
the insolubility of the lead salts of the solid acids, and the ready solubility
of those of the liquid acids, in ether. Special methods serve for the separa-
tion of the individual solid acids and the individual liquid acids.

1. Separation and Determination of the Solid and Liquid Acids.
For such separation Tortelli and Ruggeri's method is used :

20 grams of the fat are saponified in the usual way (see Saponifi-
cation), the soap being dissolved in water and the solution neutralised
towards phenolphthalein with acetic acid. Meanwhile, 300 c.c. of 7%
neutral lead acetate solution are heated in a conical flask and when this
liquid reaches the boil the soap solution is run into it in a thin stream, the
lead solution being kept stirred. The flask is then immersed in cold watere constant ester number